Pumping unit



Sept. 1, 1959 A. R. GRAD 2,901,975

PUMPING UNIT Filed Dec. 30, 1954 6 Sheets-Sheet 1 I I INVENTOR ADOLF R.GRAD BMW/Mia ATTORNEY A. R. GRAD PUMPING UNIT Sept. 1, 1959 6Sheets-Sheet 3 Filed Dec. 30, 1954 A R. GRAD PUMPING UNIT Sept. 1, 19596 Sheets-Sheet 4 Filed Dec. 30, 1954 INVENTOR ADOLF R. GRAD ATTQRNEYSept. 1, 1959 A. R. GRAD I 2,

PUMPING UNIT- Filed Dec. 30, 1954 e Sheets-Sheet 5 FIG. 6

INVENTOR 29 ADOLF R. GRAD WMWg/M 'ATTORN Sept. 1, 1959 A. R. GRAD2,901,975

PUMPING UNIT I Filed-Dec. 30, 1954 6 Sheets-Sheet 6 FIG. 7

INVENTOR ADOLF R. GRAD ATTORNEY United States Patent PUMPING UNIT AdolfR. Grad, Milwaukee, Wis., assignor to The Oilgear Company, Milwaukee,Wis., a corporation of WIS- consin Application December 30, 1954, SerialNo. 478,559

1 Claim. (Cl. 103-4) This invention relates to variable displacementpumps of the radial piston type and it has as its object to provide acompact pumping unit comprising a plurality of simultaneously drivenvariable displacement pumps which are arranged in a single casing andare individually adjustable to vary their displacements independently ofeach other.

A pumping unit embodying the invention is particularly adapted forenergizing the hydraulic motors which rotate the turret and operate thegun elevating mechanism of a military tank and the unit will beexplained as employed for that purpose but it is to be understood thatit is not limited to such use. For example, pumping units embodying theinvention can just as readily be employed for energizing the hydraulicmotors which drive the traversing and elevating mechanisms ofantiaircraft batteries.

In the present tanks, the turret traversing motor and the gun elevatingmotor are energized by separate pumps which are geared together and aredriven by single electric motor. By employing a pumping unit embodyingthe invention for energizing these motors, a saving is effected ofnearly one-half the space required for the two pumps and the gearing,and the noise and power loss in the gearing is eliminated.

Another object of the invention is to provide a pumping unit comprisinga plurality of variable displacement pumps and an auxiliary pump and toconnect the pumps in circuit in such a manner that, when the pressurecreated by any variable displacement pump becomes so high that the pumpdischarges through its relief valve, the volume of hot liquid thusexhausted through the relief valve is replaced by an equal volume ofcooler liquid supplied by the auxiliary pump.

Other objects and advantages will appear from the following descriptionof the pumping unit shown in part in detail and in part schematically inthe accompanying drawings in which the views are as follows:

Fig. 1 is a longitudinal section through a pumping unit in which theinvention is embodied, the plane of the view being indicated by the line11 of Fig. 2.

Fig. 2 is a transverse section through one of the variable displacementpumps, the plane of the view being indicated by the line 22 of Fig. 1.

Fig. 3 is a transverse section through the other variable displacementpump, the plane of the view being indicated by the line 33 of Fig. 1.

Fig. 4 is a view, partly in full and partly in section, taken on theirregular line 4-4 of Fig. 1 but drawn to a larger scale and showing theface of the valve which controis the flow of liquid to and from the pumpshown in Fig. 2, a part of the pump casing being broken away to exposethe ends of the channels through which liquid flows to and from thepump.

Fig. 5 is a sectional View taken on the irregular line 55 of Fig. 4 toshow the hold-up motors which keep the face of the valve shown in Fig. 4in contact with an end of the cylinder barrel shown in Fig. 1.

Fig. 6 is a view, partly in full and partly in section, taken on theline 6-6 of Fig. l but drawn to a much larger scale and showing the faceof the valve which controls the flow of liquid to and from the pumpshown in Fig. 3, a part of the end head of the pump casing being brokenaway to expose portions of the channels through which liquid flows toand from the pump.

Fig. 7 is a sectional view taken on the line 77 of Fig. 6 to show thehold-up motors which keep the face of the valve shown in Fig. 6 incontact with an end of the cylinder barrel shown in Fig. 1.

Fig. 8 is a diagram of the hydraulic circuit of the pumping unit when itis connected to hydraulic motors.

The pumping unit includes two reversible, variable displacement, radialpiston pumps P1 and P2 which have been shown in Fig. 1 as being ofdifferent sizes but they may be the same size. Pumps P1 and P2 arearranged within and carried by a casing comprising a body 1 which isopen at its rear end and has an integral front wall 2, an end head 3which closes the rear end of body 1 and a support 4 which is fastened tothe front end of body 1 and is adapted to support an electric motor (notshown) for driving the pumping unit. Casing. 1--4 may be mounted in anydesired position and the directional terms used hereinafter refer to thedrawings:

and not to the mounted position of the pumping unit.

Pump P1 and pump P2 are reversible variable displacement pumps eachhaving radial pistons in a rotat-- able cylinder barrel. The pistoncontaining barrels for the two pumps are constructed and arrangedcoaxially of each other for rotation with a driving shaft. As shown, thecylinder barrels for the two pumps are preferably of different size andare integral with each other and comprise a first barrel portion hereindesignated cylinder barrel 5 and a second barrel portion hereindesignated cylinder barrel 5 for pumps P1 and P2 respectively.

Pump P1 includes a cylinder barrel 5 having a bank of pistons andcylinders arranged radially therein. As shown in Figs. 1 and 2, cylinderbarrel 5 has a plurality of cylinders 6 arranged therein in two annularrows and a piston 7 fitted in each cylinder. One cylinder in each of thetwo rows communicates with a fluid channel or passage 8 which extendsfrom the inner row of cylinders through the left end of cylinder barrel5 and, during rotation of cylinder barrel 5, it registers first with oneand then the other of two arcuate ports 9 and 10 formed in an annularflat valve 11 which is held against the left end of cylinder barrel 5 aswill presently be explained. Cylinder barrel 5 is rotated by a driveshaft 12 which has been shown as being formed integral therewith. Shaft12 extends through valve 11 and through front wall 2 into motor support4 for connection to an electric motor or other prime mover not shown.

Reciprocation of pistons 7 during rotation of cylinder barrel 5 iseffected by a thrust member comprising a slide block 15, which isslidable transversely of the axis of cylinder barrel 5 and is restrainedfrom movement in any other direction, and a ball bearing 16 which isfitted in slide block 15 and has two oppositely inclined annular bearingsurfaces formed upon its inner face to engage the beveled outer ends ofpistons 7. Thus the inner race of ball bearing 16 provides thrust member15 with an annular internal bearing surface in contact with the outerends of the pistons 7.

In practice, slide block 15 is mounted between two roller bearings but,in order to simplify the drawing,

it has been shown in Fig. 2 as having a wear plate 17 fixed to its upperand lower ends adjacent to each of its corners and closely fittedbetween two bearing plates 18 and 19 which are fixed to the upper andlower walls 1 of casing body 1.

and an annular bushing 20 is fitted in the bored, portion, 1 of body- 1and holds slide block 15 in contact. with.

the inward extending portions of the wall of, bod y 1'.

The arrangement is suchthat thrust member 15-16 is free tomove in ahorizontal plane transversely of cylinder barrel 5. but is preventedfrom moving inany other direction. Bushing has fixed therein the outerrace. of a; radial thrust bearing 21 which rotatably supports; cylinderbarrel 5 and has its inner race; fixed; thereon.

Slide block 15 has formed integral therewith on, one of its sides a pairof spaced apart lugs. 22; of' which onlyone appears in the drawing Lugs22 have arranged therebetween and connected thereto bya pin 23' a link2.43 for connecting slide block 15 to a controlv 25v which is adapted toshift slide block 15 in one direction or the other to vary thedisplacement of pump P1 and which is mounted upon a support 26 formedintegral with oasing body 1. Since the control used in practice is verycomplicated, pump P1 has been shown as being: pro.-

vided with a simple hydraulic. control 25 which will be presentlydescribed but the control may be either elec.-. trical or mechanical.

As shown in Figs. 4 and 5, valve 111 has arranged therein two hold-upmotors 27, each of which. communicates with port 9: through a passage28, and two hold-up motors 29 each of which communicates with port 10through a passage 30.

Hold-up motors 27 and 29 are alike and each includes a cylinder 31 whichis formed in valve 11-, a tubular piston 32 which is fitted in cylinder31, an annular seal ing member 33 which is arranged between piston32.,and

the inner face of front wall 2, and a spring 34' for ini' tially urgingvalve 11 against cylinder barrel 5 and for initially urging piston 32against sealing member 33 and member 33 against wall 2. When pump P1 is:creating pressure, the hold-up motors on the discharge side of the pumpwill urge valve 11 against cylinder barrel 5' with a force proportionalto pump pressure, and if thereis any pressure in the liquid supplied tothe pump, the hold-up motors on the intake side of the pump will urgevalve 11 against cylinder barrel 5 with a force portionalto thatpressure, thereby preventing the pressure in the liquid between thevalve and the cylinder barrel from moving the valve away from thecylinder barrel. The contacting surfaces of pistons 32 and members 331are made spherical and smooth and the contacting surfaces of members 33and front wall 2 are made flat and smooth to provide substantiallyliquid tight seals between pistons. 32' and. front plate 2'.

Each hold-up motor 27 communicates through a. passage35 with a mainpassage 36 which is formed in front wall 2 and is adapted to beconnected at one end thereof to one side of a hydraulic circuit,.andeach hold-up motor 29 communicates through a passage 37 with. a mainpassage 38' which is formed in front wall 2 and is adapted to beconnected at one end thereof to the other side of the hydraulic circuit.Main passages 36 and 38. are i also connected totwo check valves 39 and40 respectively and to two relief valves 41 and 42, respectively. Checkvalves 39 and 46 are both connected to a supply channel 43 and reliefvalves 41 and 42 are connected to an exhaust channel 44 as shown in Fig.8.

The arrangement is such that, when slide block 15 is in its central orneutral position, bearing 16 is concern trio with cylinder barrel 5 androtation of'cylinder barrel 5 will not efiect reciprocation of pistons7. But when slide block; 15 is olfset to one side of its neutralpositionand cylinder-barrel 5 is rotated in a given direction thepistons 7 above the horizontal center line of cylinder barrel 5 will beforced progressively inward by bearing 16 and will. eject liquid fromtheir cylinders 6. through passages 8, valve port 9, passages 28,hold-up motors 27 and passages 35 into main passage 36 and bearing 16will permit the pistons 7 below the horizontal center line to moveprogressively outward and liquid to flow into their cylinders 6 throughpassages 8, valve port 10, passages 30, hold-up motors 29, and passages37 from main passage 38.

Conversely, when slide block 15. is offset to the other side of itsneutral position and cylinder barrel 5 is. rotated in the samedirection, the pistons 7 below the horizontal center line of cylinderbarrel 5 will be forced progressivel-y inward by bearing16-andwill ejectliquid from their cylinders 6 through passages 8, valve port 10,passages 30, hold-up motors 29 and passages 37 into main passage 38 andbearing 16 will permit the pistons 7 above the horizontal center line tomove progressively outward and liquid to flow into their cylindersthrough passages 8, valve port 9, passages 28, hold-up-motors- 27 andpassages 35 from main passage 36;.

Pump PI will thus deliver liquid in a direction and at a rate determinedby the direction and distance slideblock 15 is offset from its neutralposition and the pressure in the hold-up motors on each side of the pumpwill urge valve 11 against the end of cylinder barrel 5"Wll'b. a forceproportional to the pressure prevailing in that side of the pump.

The adjacent ends of valve ports 9 and 10 are spaced apart a distanceequal to the diameter of a passage 8 to providetherebetween two bridges45 and 46 which prevent one valve port from communicating with the othervalve port as the end of a passage 8 moves from one port to the other,When the pump is performing useful Work, the pressure in the passage 8in communication with the valve port 9 or 10 which at that time is thedischarge port will. berelatively high while the pressurein the passagesSi in communication with the other or intake port will be much lower ornegative. Since thereis an odd number of passages 8, the numbercontaining liquid under pressure will vary between odd and even eachtime a passage 8 crosses one of the bridges, thereby causing a variationin the blow off force which tends to move valve 11 away from cylinderbarrel 5.

In order to compensate for the variations in blow-oh force, at least onebalancing motor is arranged behind each of bridges 45 and 46 to assistthe hold-up motors in holding valve 11 in contact with cylinder barrel5'. As shown in Fig. 5, each balancing motor includes a cylinder 47,which is formed in valve 11 behind one of the bridges, and a piston 48which is fitted. in cylinder 47 and engages front plate 2. A small hole49 extends from cylinder 47 through the. face of valve 11 so that onehold-up motor is energized each time a passage 8 passes from thedischarge port on to one bridge and the other balancing motor isenergized each time a passage 8 opens to the discharge port.

Pump P2 is substantially the same as pump P1 except that it has beenshown as being smaller. Therefore, corresponding parts have beenindicated by corresponding reference numerals with the exponent aadded-to the reference numerals applied to pump P2 so that only a briefdescription of pump P2 is necessary.

Pump P2 includes a cylinder barrel 5 which has been shown as being apart of cylinder barrel 5. Cylinder barrel 5 has a bank of cylinders 6and pistons 7 arranged radially therein in two annular rows and an equalnumber of channels or passages 8 formed therein with each passagecommunicating with one cylinder in each. of the two rows and extendingthrough the right end. of. the cylinder barrel to register duringrotation of the cylinder barrel first with one and then. the other oftwo arcuate valves ports 9 and 10 formedin a flat valve 11 which engagesthe end of cylinder. barrel 5 Reciprocation of pistons 7 during.rotation of cylinder barrel 5 is effected by a thrust member comprisinga slide block 15 which is slidable transversely of the axis of cylinderbarrel 5 and is restrained from movement in any other direction, and aball bearing 16 which is fitted in slide block 15 and has two oppositelyinclined annular bearing surfaces formed upon its inner face to engagethe beveled outer ends of pistons 7 In practice, slide block 15 ismounted between two roller bearings but, in order to simplify thedrawing, it has been shown in Fig. 3 as being slidably mounted inbushing 20 by means of wear plates 17 and bearing plates 18 and .19.Slide block 15 is restrained from axial movement by a spacer ring 51,which is arranged between it and the outer race of ball bearing 21, andby a flanged spacer ring 52 which is arranged between end head 3 andbushing 20.

Slide block 15 has formed integral therewith on one of its sides a pairof spaced apart lugs 22 of which only one appears in the drawing. Lugs22 have arranged therebetween and connected thereto by a pin 23 a link24* for connecting slide block 15 to control 25 which is adapted toshift slide block 15 in one direction or the other to vary thedisplacement of pump P2 and which is mounted upon support 26. For thepurpose of illustration control 25 has been shown as being of a simplehydraulic type and will be presently described. When cylinder barrel 5is rotated, pump P2 will deliver liquid in a direction and at a ratedetermined by the direction and distance slide block 15 is offset fromits neutral position in the same manner as does pump P1.

As shown in Figs. 6 and 7, end head 3 has arranged therein two hold-upmotors 57, each of which communicates with valve port 9 through apassage 58 formed in valve 11*, and two hold-up motors 59 each of whichcommunicates with valve port through a passage 60 formed in valve 11*.Hold-up motors 57 and 59 urge valve 11 against the end of cylinderbarrel 5 and are assisted in doing so by two balancing motors eachincluding a cylinder 47 which is formed in valve 11 and has a small hole49* extending therefrom through the face of the valve plate at a pointbetween the adjacent ends of the two valve ports, and a piston 48 whichis fitted in cylinder barrel 47 and engages end head 3. Hold-up motors57 and 59 and balancing motors 47 18 functions in the same manner as thehold-up motors 27 and 29 and balancing motors 47-48 respectively.

Hold-up motors 57 and 59 are alike and each includes a cylinder 61 whichis formed in end head 3, a tubular piston 62 which is fitted in cylinder61, an annular sealing member 63 which encircles a passage 58 or 60 andis arranged between end head 3 and the rear face of valve 11 a chamber64 which is formed in end head 3 and is concentric with and slightlysmaller than cylinder 61, and a keeper ring 65 which is arranged betweenthe end of piston 62 and the shoulder between cylinder 61 and chamber 64to prevent piston 62 and sealing member 63 from moving a substantialdistance away from valve 11 when there is no pressure in chamber 64. Anysubstantial movement of valve 11 relatively to end head 3 is preventedby two pins 66 (Fig. l) which connects valve 11 to end head 3 and hassufficient clearance to permit valve 11 to adjust itself to the end ofcylinder barrel 5 The contacting surfaces of valve 11 and sealingmembers 63 are made flat and smooth and the contacting surfaces ofmembers 63 and pistons 62 are made spherical and smooth to providesubstantially liquid tight joints between valve 11 and pistons 62.

The two chambers 64 of hold-up motors 57 communicate with a main passage67 which is formed in end head 3 and is adapted to be connected at oneend thereof to one side of a hydraulic circuit and the two chambers 64of hold-up motors 59 communicate with a main passage 68 which is formedin end head 3 and is adapted to be connected at one end thereof to theother side of the hydraulic circuit.

Main passages 67 and 68 are also connected to two check valves 69 and 70respectively and to two relief valves 71 and 72 respectively. Checkvalves 69 and 70 are both connected to supply channel 43 and reliefvalves 71 and 72 are both connected to exhaust channel 44 as shown inFig. 8.

The arrangement is such that, when the cylinder barrel is rotated andslide block 15 is offset to one side of its neutral position, thepistons in cylinder barrel 5 above its horizontal centerline will beforced progressively inward and the pistons below the horizontalcenterline will move progressively outward. The inward moving pistonswill eject liquid from their cylinders through passages 8 valve port 9passages 58, hold-up motors 57 and chamber 64 into passage 67 and thecylinders containing outward moving pistons will be supplied with liquidthrough their passages 8 valve port 19 passages 60, hold-up motors 59and chambers 64 from main passage 68. When slide block 15 is offset tothe other side of its neutral position, the flow of liquid will bereversed.

Since valves 11 and 11 are prevented from moving away from the cylinderbarrel by the hold-up motors, the pressure in the liquid between valve11 and the cylinder barrel urges the cylinder barrel toward the rightwhen pump P1 is creating pressure and the pressure in the liquid betweenvalve 11 and the cylinder barrel urges the cylinder barrel toward theleft when pump P2 is creating pressure but one pump may be creating amaximum pressure when the other pump is not functioning so that thecylinder barrel is urged axially by the full force of the liquid betweenone of the valves and the cylinder barrel.

In order to prevent any axial movement of the cylinder barrel, shaft 12is rotatably supported by a double row preloaded axial thrust bearing 76(Fig. l) which has its inner race mounted upon shaft 12 and rigidlysecured against axial movement in either direction and its outer raceclosely fitted in support 4 and rigidly secured against axial movementin either direction.

if the pump is of substantial size, bearing 76 may be of the type havingtwo rows of tapered rollers with the rollers in one row taperedoppositely to the rollers in the other row. But if the pump is small,bearing 76 may be of the type having two rows of balls as shown in whichcase the inner and outer races are so proportioned that the balls in onerow bear heavily against the outer race at the right of the center lineof the row and bear heavily against the inner race at the left of thecenter line of the row and the balls in the other row bear heavilyagainst the outer race at the left of the center line of the row andbear heavily against the inner race at the right of the center line ofthe row so that, when the bearing is pressed into support 4, one row ofballs tends to urge shaft 12 in one direction and the other row of ballstends to urge shaft 12 in the opposite direction. The bearing is thuspreloaded and positively prevents any axial movement of the cylinderbarrel.

In order to supply cool liquid to either of pumps P1 and P2 when itdischarges liquid through its relief valve and in order to provideliquid for supercharging pumps P1 and P2 and for control purposes, thepumping unit is provided with an auxiliary pump P3 which has been shownas a gear pump having its driving gear 77 fixed on shaft 12 and itsidler gear 78 journaled in front wall 2 and in support 4.

As shown in Fig. 8, pump P3 draws liquid from a reservoir 79 anddischarges it into supply channel 43 at a rate in excess of requirementsand the excess liquid is exhausted into reservoir 73 through a channel80 having connected therein a relief valve 81 which enables pump P3 tomaintain a constant low pressure in channel 43.

Channel 43 is formed in part by external piping and in part by passagesformed in the casing of the pumping unit. It is connected to checkvalves 39, 40, 69 and 70, as

7. previously explained, and it is also connected to controls 25 and 25which will now be described.

For the purpose of illustration, control 25 has beenshown in Fig. 2 asincluding a cylinder 82 which is fastened to support 26, a differentialpiston 83 which is fitted in cylinder 82. and is connected by a pin 84to the end of link 24 which is arranged within a bore 85 extendingaxially through piston 83', and a pilot valve 86 which is fitted in bore85 and may be shifted manually but in practice it is shifted by anelectronically controlled apparatus not shown. Valve 86. is also fittedin a bore 87 which is formed in the end head of cylinder 82 and has aport 88 formed in the wall thereof.

Piston 83 has an annular groove or port 89 formed therein around bore 85and a passage 90 extending from port 89 through the outer end of piston83. Pilot valve 86 is reduced in diameter near its inner end to providea cannelure 91 and a valve piston 92 which is the same width as port 89so that a slight movement of valve 86 in one direction or the other willopen port 89 either to cannelure 91 or to the inner part of bore 85.Cannelure 91 is connected to port 88 by a passage 93 formed within valve86.

Liquid for energizing control 25 is. supplied by pump P3v throughchannel 43 a branch of which is connected to the inner end of cylinder82 by a passage 94 formed in the wall of cylinder 82. and is connectedto port 88 by a passage 95 formed in the end head of cylinder 82 so thatcannelure 91 and the inner end of, cylinder 82 are constantly suppliedwith liquid at the same pressure.

Piston 83 has the inner part thereof reduced in diameter and extendingthrough the inner end wall of cylinder 82 and it is so proportioned thatthe piston area upon which the liquid in the outer end of cylinder 82'acts is considerably greater than and preferably is twice as great as.the piston area upon which the liquid in the inner end of cylinder 82.acts.

The arrangement is such that, when pilot valve 86 is moved inward agiven distance, liquid will flow from channel 43 through passage 95,port 88, passage 93, cannelure 91, port 89 and passage 9% into the outerend of cylinder 82 and cause piston 83 to move slide block 15 toward theleft and to eject liquid from the inner endof cylinder 82 throughpassage 94- into passage 95 until piston 83 has moved exactly the samedistance that valve- 86 was moved and then valve piston 92 will coverport 89' and further movement will cease.

Conversely, when pilot valve 86 is moved outwarda given distance, liquidwill fiow from channel 43 through passage 94 into the inner end ofcylinder 82 and cause piston: 83 to move slide block 15 toward the rightand to eject liquid from the outer end of cylinder 82 through passage 9;port 89 and bore 85 to exhaust until piston 83. has moved exactly thesame distance that valve 86 was moved and then valve piston 92' willcover port 89 and further movement will cease.

Control 25 (Fig. 3) is identical to control 25, like parts of the twocontrols have been designated by like reference numbers, and control 25*functions to control pump P2. in the same way that control 25 functionsto control pump Pi. Therefore, a detailed description thereof isunnecessary.

Pump P1 has been shown inFig. 8.as being connected to a rotary hydraulicmotor M1 by two channels 96 and 97 which form therewith a closedhydraulic circuit and pump P2. has been shown as being connected toa-recipro eating; hydraulic, motor M2. by two channels 98 and 99 whichform therewith a closed hydraulic circuit but either pump P1 or pump.P2v may be employed. for energizing either a rotary motor or areciprocating, motor or for other purposes. The two pumps, the twomotors and the interconnecting channels thus constitute two hydraulictransmissions which function independently of each other.

Channels 96, 97, 98 and 919 are formed in part by external piping and inpart by passages formed in the 8. casing of the pumping unit.Specifically, channel 96 in cludes passage 36 (Fig. 4) to which checkvalve 39 and relief valve 41 are connected, channel 97 includes a mainpassage 38 (Fig. 4) to which check valve 48' and relief valve 42 areconnected, channel 93 includes main passage. 67 (Fig. 6') to which checkvalve 69 and relief valve 7 are connected, and channel 99 includespassage 68 (Fig. 6) to which check valve 70 and relief valve 721 are.connected.

The arrangement is such that, when the pumping: unit is operating, pumpsP1 and P2 are supercharged by pump P3. That is, any leakage inthetransmissions is made up by pump P3 which maintains a low pressure atthe intake of pumps P1 and P2 when those pumps are dis=- charging liquidin either direction and it maintains a low pressure at bot-l1 sides ofeither pump P1 or F2 when the pressure created by that pump is less thanthe pressure created by pump P3.

In a transmission in which the pump and motor is connected into aclosedor substantially closed hydraulic circuit, it is customary to connectthe relief valves be, tween the two sides of the circuit so that whenthe pump discharges through a relief valve in response to the load onthe motor becoming excessive, the liquid discharged through thereliefvalve flows directly to the intake of the pump,. thereby causing theenergy dissipated at the reliefvalve to heat the liquid in the circuit.

When pump P1 or P2 creates a pressure in excess of a predetermined.maximum, the pump will discharge liquidthrough the relief valveconnected to the dischargeside of the pump and an equal volume of liquidwill flow from supply channel 43 into the intake side of the pumpthrough the check valve connected thereto. For example, if pump P1should discharge liquid through relief valve 41, it will be suppliedwith liquid through: check valve 40:

Since nearly all of the liquid discharged by pump P3 is exhaustedthrough relief valve 81 during the greater part of the time that thepumping unity is in operation, it is desirable that the. volumetriccapacity of pump P3 be considerably less than the capacity of pump P1 asotherwise the liquid in reservoir 79 would become unduly heated by theheat generated at relief valve 81.

If pump P1 or P2 should exhaust liquid through oneof. its relief valveswhen its slide block was so. adjusted that the rate of exhaust was nogreater than the rate at which. pump P 3 delivers the liquid intochannel 43:, all of the. liquid exhausted through the relief valve wouldflow through channels 44 and and relief valve 81 into reservoir 7-9 andan equal volume of cool liquid would be supplied by pump P3 to theintake side of pump P1 or P2 through the check valve connected thereto.But if pump P1 or P2 was so adjusted that the rate at which it exhaustedliquid through its relief valvewas greater than the. volumetric capacityof pumpv P3, enough of the exhausted liquid to make up the differencebetween the.

volumeof liquid exhausted into. channel 44 and thevolume discharged bypump P3 would fiow through channel 80 into channel 43 and the remainderwould be exhausted through relief valve 81 in which case pump P1 or P2would. be supplied. with a mixture of hot and cool liquid through thecheck valve. connected to its intake side.

Thepumping unit described herein may be modified in various ways withoutdeparting from the scope of the invention which is hereby claimed. asfollows:

A pumping unit comprising a casing, a drive shaft journaled in saidcasing, two reversible variable displace.- ment pumps. arranged withinsaid casing. and connected tosaid shaft to. be driven thereby, two mainchannels connected to opposite. sides of each of said variabledisplacement' pumps for connecting said variable displacement pumps toseparate hydraulic motors, an exhaust.-

to that pump creating a pressure in excess of a predetermined maximumduring pump delivery in either direction, an auxiliary constantdisplacement pump connected to said shaft to be driven thereby, areservoir connected to the intake of said auxiliary pump for supplyingrelatively cool liquid thereto, a supply channel connected to the outletof said auxiliary pump, and a check valve connecting each of said mainchannels to said supply channel to enable said auxiliary pump to supplyto the intakes of said pumps an amount of relatively cool liquid fromsaid reservoir to replace the relatively hot liquid discharged into saidexhaust channel, a by-pass channel connecting the outlet of saidauxiliary pump to a relatively low pres- 10 sure relief valve thatdischarges fluid to said reservoir, and said exhaust channel connectedto said by-pass channel so that all said relief valves for said pumpsare connected to said reservoir through said low pressure relief valve.

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